1. Field of the Invention
The present invention relates to a spacer type optical fiber cable containing coated optical fibers which is necessary for the construction of an optical communication network. More particularly, the present invention relates to a spacer type optical fiber cable suitable for in-building installation where vertical installation is also required.
2. Description of the Related Art
Optical fiber cables have been developed which comprise coated optical fibers fitted to a grooved spacer in a high density. For example, a spacer type optical fiber cable is known which comprises coated optical fibers bundled with a yarn, string, or the like and fitted to a grooved spacer, as shown in JP-A-U-59-181403 and JP-A-62-258411. (The terms "JP-A-U" and "JP-A" as used herein mean an "unexamined Japanese utility model publication" and an "unexamined Japanese patent publication", respectively.)
The conventional spacer type optical fiber cables of the above kind have the following problems because the degree of packing of coated optical fibers in the grooves of the spacer has not especially been taken in account. The "degree of packing" herein is calculated for each groove and defined by the following equation.
Degree of packing (%)=[(total sectional area of the coated optical fibers)/(sectional area of that space in the groove where an coated optical fiber can move)].times.100
It is a matter of course that the sectional area of that space in a groove where an coated optical fiber can move includes the sectional area occupied by the coated optical fibers. Consequently, when coated optical fibers alone have been disposed in a groove having a sectional area of S and the total sectional area of the coated optical fibers is F, then the degree of packing is (F/S).times.100.
In the case where an optical fiber cable containing coated optical fibers disposed in grooves is subject to low-temperatures, e.g., in winter, the coated optical fibers become longer relative to the cable due to a difference in the coefficient of linear expansion between the material constituting the cable and the coated optical fibers, and hence tend to meander within the grooves. Namely, a phenomenon occurs in which the coated optical fibers come to have an excess of length. In general, this kind of optical fiber cable is manufactured so that the coated optical fibers have a length in excess of the minimum necessary length in some degree. However, if the coated optical fibers in an optical fiber cable having too high a degree of coated optical fiber packing come to additionally have such an excess of length, the coated optical fibers come to have a reduced pitch of meandering in each groove because of the small space and, hence, the radius of curvature of the bends of the coated optical fibers is reduced. Since the meandering pitch is not always constant, the parts in which the excess of length is concentrated have an exceedingly reduced radius of curvature and this results in an increased bending loss and sometimes leads to breakage of the coated optical fibers.
On the other hand, this kind of optical fiber cable is often used for in-building vertical installation. In this application, if the degree of packing of the coated optical fibers is too low, then the coated optical fibers with some excess length readily shifts downward to generate an area where the execss length is concentrated, and this also results in an increased bending loss and sometimes laeds to breakage of the coated optical fibers.
In JP-A-U-59-181403 cited above, there is no disclosure of these problems, and no means for solving the same is given. In JP-A-62-258411 discloses a technique in which coated optical fibers are fitted into helical grooves while applying tensile stress thereto. However, the cable containing coated optical fibers disposed under tensile stress has a problem that the tension imposed on the coated optical fibers increases when the cable is bent or as the temperature of the atmosphere changes.
Furthermore, air-core type cables in which the grooves are not filled with a jelly have a problem that yarns coarsely wound around the coated optical fibers get loose especially at an end part, although cables in which the grooves are filled with a jelly are free from the problem of the yarns readily getting loose.